Climate change is already contributing to mass coral bleaching events – and now researchers have found that the mountains of plastic in our oceans could be infecting corals with diseases.

The research, published in the journal Science, found that 89 per cent of corals surveyed were in contact with plastic and contaminated with disease. By contrast, only four per cent of corals without plastic entangled in them had signs of disease. This study is the first clear link between coral health and increasing interaction with plastic waste.

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Joleah B. Lamb, co-author of the study and a researcher at Cornell University, found that plastic waste harbours bacteria that can infect the coral by cutting into the upper skin of the reefs and passing on diseases. Additionally, the plastic suppresses the oxygen and light levels for the coral, creating ideal environments for the spread of bacteria.

The team of researchers, based in the US, Australia, Indonesia, Myanmar and Canada, surveyed 159 reefs in the Asia-Pacific region, estimating that 11.1 billion pieces of plastic are currently entangled on reefs across the region. The researchers predict this number will increase 40 per cent by 2025.

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"Healthy coral reefs are essential for marine biodiversity, providing a barrier for waves and preventing the coastline from eroding into the ocean," says Lamb. Ornate corals, with complex branches, are eight times more likely to be entangled with plastic due to their shape. These corals are particularly important for small fish that rely on corals to survive.

The team looked at the diseases most common in corals and found that diseases such as white syndrome increased by 17 per cent if the reef was in contact with plastic. The disease, which creates a white band around healthy coral, is caused by the the pathogenic bacteria Vibrio.

A great star cora infected with black band disease

Wild Horizons/UIG via Getty Images

"These bacteria are common in the marine environment but under some conditions they can cause disease in corals. Other studies have found that there are a lot of kinks and pores for these bacteria to colonise plastic waste and latch on to it," says Lamb. Plastic that becomes entangled in coral may be spreading these bacteria when it hits the reefs. "It is like cutting yourself with a really dirty knife," she adds.

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Black band disease was also associated with the presence of plastic. "This disease is essentially when a mat band of bacteria and other microorganisms form and erode away the tissue of the coral," says Lamb. This tends to occur in very low light and at low oxygen levels.

This means that when plastic lands on top of the coral, micro-environments form around the plastic where microorganisms and bacteria can multiply and enjoy the low light, low oxygen surroundings. "It is also stressing the coral because it no longer gain access to light and so cannot access its energy resources," says Lamb.

But there is some hope. The surveyors found the highest amount of plastic in Indonesia and the lowest in Australia. This confirms previous studies which found that low income countries such as Indonesia do not have the proper facilities to successfully reduce plastic waste. "It shows that if we can start to enforce and reduce the amount of plastic that is entering the ocean, like Australia is doing, then we should be able to see a reduction on the reef and the level of mortality that is occurring from disease," Lamb adds.